(175a) Kinetics, Mechanism, and Structure Requirements for Selective Terminal Oxidation of Linear Alkanes on Mn-Modified Microporous Catalysts

Selective oxidation of terminal C-H bonds in linear alkanes (RH) to oxygenates [e.g. alcohols (ROH), aldehydes/ketones (R(-H)=O) and acids] is a useful route to intermediates for fine chemicals, polymers and pharmaceuticals, but remains a significant challenge. We report here a systematic study of the oxidation of n-hexane by O₂ on Mn-exchanged zeolites (Mn-ZSM-5, Mn-ZSM-57, Mn-ZSM-58 and Mn-MOR) and Mn-containing aluminophosphate materials (MnAPO-5 and MnAPO-18) with different channel structures to explore the effects of spatial constraints on rate and regioselectivity. n-Hexane oxidation proceeds with hexyl hydroperoxides (ROOH) as reactive intermediates on all materials. Formation rates for ROH and R(-H)=O were first order in ROOH concentration, as previously observed for cyclohexane oxidation on MnAPO-5. This suggests that decomposition of ROOH on redox-active Mn sites is the kinetically-relevant step to form Mn-bound reactive intermediates such as ROO, RO, and R, which further react with n-hexane forming primary products and also regenerate ROOH intermediates in heterogeneous chain transfer steps. Pseudo-first-order rate constants for ROOH decomposition to form ROH, R(-H)=O, and acids were 2.5, 1.4, 0.41, 0.31 and 0.38 mol (mol Mn-h)^-1 (mM ROOH)^-1 on Mn-ZSM-5, Mn-ZSM-57, Mn-MOR, MnAPO-5, and MnAPO-18 catalysts, respectively. In contrast, product formation rates on Mn-ZSM-58 gave a stronger than first order ROOH dependence as in the case of non-catalytic reactions, suggesting that small windows (0.36 nm) in Mn-ZSM-58 restrict entry of reactants. These results suggest that n-hexane oxidation on these microporous materials is strongly influenced by the environment of the active Mn sites. The terminal selectivity of O-attachment during n-hexane oxidation was also influenced by spatial constraints within microporous channels. Zeolites with 10-membered rings gave higher terminal selectivity (24% on Mn-ZSM-5; 14% on Mn-ZSM-57) than the 8- and 12-membered ring structures (8-10%), which showed selectivities similar to those for non-catalytic reactions (8%) and in agreement with linear free energy relationships based on the relative energies of the various C-H bonds in n-hexane. Addition of 10-ring and 12-ring H-form zeolites (e.g. H-ZSM-5, H-ZSM-57, and H-MOR) to scavenge ROOH intermediates and preventing unselective non-catalytic pathways indicates that the main role of confined Mn sites is to decompose ROOH and catalyze heterogeneous chain transfer processes. Density functional theory simulations showed that ZSM-5 channel intersections tend to orient n-hexane molecules so as to favor contacting active Mn-species with terminal C-H bonds over secondary C-H bonds in n-hexane.